[Bug libc/14578] New: fchmodat(..., AT_SYMLINK_NOFOLLOW) returns ENOTSUP on non-symlinks

["mgorny at gentoo dot org" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14578

             Bug #: 14578
           Summary: fchmodat(..., AT_SYMLINK_NOFOLLOW) returns ENOTSUP on
                    non-symlinks
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: unassigned@sourceware.org
        ReportedBy: mgorny@gentoo.org
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


Tested with glibc-2.15, the relevant code verified to be the same in current
git.
CHOST: x86_64-unknown-linux-gnu
Kernel: 3.5.0 (vanilla)

Whenever fchmodat() is called with AT_SYMLINK_NOFOLLOW on Linux, it
unconditionally fails with ENOTSUP. The following snippet from
sysdeps/unix/sysv/linux/fchmodat.c is responsible for that:

#ifndef __NR_lchmod     /* Linux so far has no lchmod syscall.  */
  if (flag & AT_SYMLINK_NOFOLLOW)
    {
      __set_errno (ENOTSUP);
      return -1;
    }
#endif

POSIX.2008 however specifies[1] that:

[EOPNOTSUPP]
  The AT_SYMLINK_NOFOLLOW bit is set in the flag argument,
  path names a symbolic link, and the system does not support changing
  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  the mode of a symbolic link.

In other words, I believe that fchmodat() should only return ENOTSUP if
AT_SYMLINK_NOFOLLOW is requested on a symbolic link. On regular files, it
should just change the mode.

[1]:http://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html

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